1. Field of the Invention
The present invention relates to a steady rest for centering a rotationally symmetrical workpiece in a space.
2. Description of the Prior Art
Steady rests of this kind have been used for decades for supporting rotationally symmetrical workpieces on machines tools. In particular, with heavy and long rotationally symmetrical workpieces, it is necessary to secure them centrally in the space by means of several steady rests spaced apart from one another in order to compensate for the bowing of the workpiece. The workpieces undergo bowing under their own weight, especially when rotating for machining involving metal cutting.
In the course of a plurality of machining steps, furthermore, additional machining forces are created which act on the workpieces and by means of which the central positioning of the workpiece could be changed. The steady rests of prior art are therefore intended to prevent the machining forces which act on the workpieces from giving rise to positional changes of this kind.
Machining involving metal cutting reduces the weight of the workpieces, as a result of which there is often also a change in position, with the effect that the steady rests have to be reopened after a particular reduction in the weight of the workpieces in order to position the workpieces in the space so that the lengthways axis in the clamped workpiece runs along a straight line without deviating from the central axis.
A steady rest with an internal adjusting device for the central axis of the workpieces is disclosed in EP 0 554 506 61. In order to achieve this, it is necessary for the two outer steady rest arms to be moved differently in relation to one another in order to achieve the vertical alignment of the workpiece. Horizontal centering of the workpiece is performed jointly with all three steady rest arms.
EP 0 554 506 B1 proposes an adjusting device for vertical and horizontal alignment of the steady rest arms which has an extremely complex design.
It has proven to be a disadvantage of an adjusting device of this kind that it is very expensive to manufacture, and that the operation and mode of function of the adjusting device is complicated, such that specialist personnel are required who have the corresponding experience for undertaking the vertical and horizontal alignment of the steady rest arms.
DE 60208835 T2 or EP 1 302 275 A2 disclose a vertically adjustable steady rest, the steady rest arms of which have temporally discrete contact points on the contact surface of the workpiece by means of rocker and sliding elements which are mounted on the housing halves.
The sliding and rocker elements, also referred to as tappets, adjust the positions of the two outer steady rest arms synchronously with one another in such a way that the control pins projecting from the steady rest arms enter into active contact with the contact surfaces either simultaneously or with a time offset, and are therefore moved in the direction of the workpiece. Moving the sliding elements sideways to the movement direction of the middle piece and the angled surface formed on it causes the aforementioned adjustment of the feed sequences of the two outer steady rest arms to take place.
Although this state of the art has proven effective for adjusting the feed movement of the steady rest arms, it is, however, necessary to produce additional components and to work holes into the middle piece or the housing in which the sliding and rocker elements are mounted in a sliding arrangement. Production of such high-precision components is extremely costly, because the sliding and rocker elements must have identical contact surfaces running at an angle. If there are fault tolerances due to tolerance deviations which are a result of the manufacturing process, then synchronous feed or adjustment of the two outer steady rest arms is not possible, because this would result in an unwanted movement of the workpiece.